999
FXUS21 KWNC 071939
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 07 2024

SYNOPSIS: By late next week, a departing frontal system is expected to increase
the potential for high winds over the northeastern contiguous U.S (CONUS).
Upstream, strong mid-level low pressure shifting into the Interior West brings
increased risk of high elevation heavy snow and high winds through the middle
of the period.  Additional surface low formation in the lee of the Rockies may
also bring another round of heavy precipitation across the central U.S., with
accumulating snowfall possible on the backside of the low across the
north-central CONUS.  Surface low pressure shifting eastward from the Bering
Sea may lead to episodes of strong winds for much of coastal Alaska.

HAZARDS

Slight risk of heavy precipitation for parts of the Great Plains, Mississippi.
Tennessee. and Ohio Valleys, Sat-Tue, Nov 16-19.

Slight risk of heavy snow across many portions of the Rockies, Fri-Tue, Nov
15-19.

Slight risk of heavy snow for the higher elevations of the Sierra Nevada range
of California, Fri-Sat, Nov 15-16.

Slight risk of high winds across many portions of the northeastern CONUS,
Fri-Sat, Nov 15-16.

Slight risk of high winds for the entire western CONUS and Central and Northern
High Plains, Fri-Tue, Nov 15-19.

Slight risk of high winds across coastal portions of Southeast Alaska, western
and southern Mainland Alaska, and the Aleutians, Fri-Tue, Nov 15-19.

DETAILED SUMMARY

FOR SUNDAY NOVEMBER 10 - THURSDAY NOVEMBER 14:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY NOVEMBER 15 - THURSDAY NOVEMBER 21: Based on the latest 500-hPa
height forecasts from the dynamical models, the week-2 hazards perspective
largely remains on track. Leading into the start of the period, both the GEFS
and ECWMF ensembles continue to favor shortwave troughing lifting out over the
northeastern CONUS, with the latter model still favoring a stronger mean
solution of this mid-level feature.  Perhaps the biggest change in the guidance
since yesterday concerns the pattern upstream, as all models have become more
robust with an amplified 500-hPa trough that encroaches the West Coast early in
the period. As much of the anomalous troughing shifts eastward into the
Interior West, there remains an increased risk of high elevation heavy snow and
high winds for many parts of the West, as well as additional surface low
formation in the lee of the Rockies which may bring heavy precipitation
accumulations to parts of the Great Plains, Mississippi, Tennessee and Ohio
Valleys towards the middle of the period.



Tied to shortwave troughing forecast early in week-2, a frontal system is still
favored to track into the Northeast and Canadian Maritimes with much of the
increased precipitation appearing to be well offshore on day 8. However, based
on 500-hPa height trends over the past few days resulting in a deeper mean low
with stronger surface pressure gradients, a slight risk of high winds remains
issued for the parts of the Mid-Atlantic and the Northeast for Nov 15-16. While
there isnt much support for ample cold air advection and a significant drop in
temperatures in the wake of the system, it is worth noting that the Great Lakes
water temperatures have been substantially above normal (and in some cases
record breaking) during the past month or so according to GLSEA/NOAA. As such,
parts of the Great Lakes look to be primed for lake-effect precipitation/snow
should any anomalously cold air work its way into the region.



Over the West Coast, the encroaching mid-level trough axis upstream is favored
to be onshore by day 8 (Nov 15), resulting in drier conditions over the Pacific
Northwest following a period of enhanced moisture flow late in week-1.  This is
reflected in both the GEFS and ECWMF based Integrated Vapor Transport (IVT)
tools, allowing the slight risk of heavy precipitation and high elevation heavy
snow hazards to time off over the Pacific Northwest in the updated outlook.  A
secondary surge of moisture is depicted in these tools, which appears to be
tied to another shortwave trough affecting the Pacific Northwest towards the
middle of the period. This is also supported in the raw daily precipitation
guidance during days 10 and 11, however Probabilistic Extremes Tools (PETs)
suggest amounts are not expected to exceed hazard thresholds, and a
corresponding hazard is not issued at this time. This renewed heavy
precipitation potential will continue to be monitored in upcoming outlooks.



While conditions appear drier in the Pacific Northwest, the position of the
mean trough axis still appears favorable for enhanced moisture flow further
south along the West Coast early in the period. The GEFS Snow Water Equivalent
(SWE) PET shows increased signals over the Sierra Nevada range, with stronger
signals over much of California in both the GEFS and ECWMF liquid equivalent
PETs. While the ECMWF ensemble is less onboard with the snow potential, a
slight risk of heavy snow is posted for Nov 15-16 over the higher elevations of
California.  Along with cooler temperatures, the favored increase in moisture
over California is expected to quell fire weather potential and provide relief
to areas affected by fire incidents in recent weeks. However, as California
heads into the wet season, it is also worth noting that any heavy precipitation
over any burn scarred areas of the state may trigger debris flows.



As the amplified troughing moves further inland over the Interior West through
the middle of the period, the combination of continued Pacific moisture with
cyclonic flow aloft over the higher elevations is expected to remain conducive
for periods of potentially heavy snow over the Rockies.  Given the stronger and
deeper troughing favored aloft, a broad slight risk area for heavy snow is
continued and expanded southward into the lower Four Corners in the updated
outlook for Nov 15-19. This expansion is supported in the GEFS SWE PET which
depicts a broader coverage of SWE values exceeding the 85th percentile
throughout much of the West. Additionally, a slight risk of high winds also
remains issued for Nov 15-19 and is also expanded southward in the updated
outlook. A moderate risk was considered for inclusion based on the pattern
aloft, but there is less support for this in PETs which show 20-30% chances for
winds exceeding the 85th percentile through the middle of week-2.



The longwave troughing pattern also supports the potential for additional lee
cyclogenesis where there is also better support and good continuity in the raw
and calibrated tools for surface low formation and the return of enhanced
precipitation to the central U.S. A slight risk of heavy precipitation remains
posted and is expanded to include additional areas of the Plains, Mississippi,
Tennessee and Ohio Valleys, based on increased signals in the PETs for amounts
exceeding the 85th percentile and the uncalibrated ECMWF for 3-day amounts
exceeding an inch.  On the backside of the surface low, accumulating snowfall
is possible over the High Plains where ECMWF ensemble depicts increasing daily
amounts in the daily means beginning on days 10 and 11 (Nov 17-18) over the
lower elevations. However, uncertainty is too high at this lead, and there is
little support for snowfall amounts exceeding hazard criteria to warrant a
corresponding heavy snow hazard at this time.



Over Alaska, anomalous 500-hPa is favored to develop over the Bering Sea, where
ensembles continue to show additional low pressure development, with a
secondary mean low forming in the Gulf of Alaska which may bring another period
of strong pressure gradients over much of southern and western Alaska. Based on
continued support in the PETs, a slight risk of high winds remains posted, and
is expanded to include the western Mainland, and the Southeast, as well as
extended through Nov 19 based on elevated signals persisting in the PETs.

FORECASTER: Nick Novella

$$